The invention is directed to a noise suppression method and circuit for an electronic device for suppressing electrostatic discharge (ESD) noise pulses which affect the electronic device and data buses connected to these devices.
Most of the problems encountered with ESD are caused by electrostatically charged human bodies touching the electronic devices, either its chassis or external cables, causing discharge currents to flow towards ground. For example, hard to solve problems occur when tape cassettes, cartridges or diskettes are inserted into data storage devices. These devices are usually mounted as subassemblies in the computer chassis.
During a discharge event, tens of amperes can flow in less than a nanosecond. An electromagnetic wave will propagate towards ground on the chassis and the cables leaving the device. Due to the very high frequencies involved, much of the prior art ESD problem solving has been directed towards proper shielding and grounding of the electronic devices. The coupling mechanisms can be divided into four parts: Direct conduction, secondary arcing, electric field, and magnetic field couplings. To solve the problems associated with electric field coupling to the circuits, it is recommended not to float the signal grounds with respect to the chassis ground or not to provide an external ground connection independent of the chassis ground to the electronic devices. A multiple grounding scheme is therefore often used for the devices inside the chassis. To solve the problems with magnetic coupling, shielding is usually recommended. However, for cables used for data buses, problems arise due to this multiple grounding scheme. When the signal ground of, say, both ends of a data bus cable is connected to the chassis, a ground loop is formed. Noise currents can then flow if the chassis potential is different at the two ends or if voltages are induced in the loop due to magnetic coupling. For a discussion of the prior art, see Ott, H. W., "Noise Reduction Techniques in Electronic Systems", 2nd ed. John Wiley & Sons, New York, 1988; and Hewlett Packard Course No. HP 11949A; "Designing for Electromagnetic Compatibility", Application Support Division, Mountain View, Calif., U.S.A., 1989.
It is known in the art that ESD protection of the data buses can be obtained by using ferrite tubes or clamped ferrite pieces around the signal cables. See Murakami, Yuichi, JEE Journal of Electronic Engineering, Vol. 8, No. 174, July 1981, page 49, FIG. 8; and Hewlett Packard Course No. HP 11949A, supra.
The function of these ferrite components is then as a common-mode inductor.
The problem with these data bus protection schemes can be divided into two parts. First, due to the leakage inductance of the common mode inductors available, the high frequency components of the differential signal currents may also be attenuated if the common mode impedance is too high. It is therefore very difficult to obtain a good protection against noise. Second, the data buses typically consist of as many as 50 signal and ground wires and a common mode inductor for all these signal lines becomes very expensive. An alternative consists of using ferrite clamps around the cables, but even those tend to be expensive parts.
It can be also mentioned here that other types of usage exists for common-mode inductors. They can be used to prevent conducted radiated electromagnetic noise from leaving the devices via cables. See, for example, Siemens Aktiengesellschaft, "EMC EMI Suppression Components, Filters", Data Book 1987/88. For signal lines, the leakage inductance can also be utilized to limit the bandwidths of the differential signal waveforms transmitted on the cables.